1. Field of the Invention
The invention is directed to a method and apparatus for detecting a substrate feature.
2. Description of the Related Art
Substrates, such as paperboard webs, are often coated with a variety of coatings or finishes. Extrusion coating is a process where a substrate, such as paperboard or a sheet of paperboard, is coated with a fluid or liquid, such as polyethylene. Conventionally the coating is placed on the substrate using extrusion methods. A typical method uses various equipment to move the substrate under an extrusion coating apparatus. The extrusion coating apparatus sprays or dispenses the coating material onto the substrate surface. Typically, the coating extends beyond at least one of the substrate's perimeters, thereby creating an “edge bead”. In many coated substrates, it is important that an edge bead exist to ensure that the entire substrate is adequately coated.
Some coating materials are difficult to control at the substrate's perimeter. These coating materials, such as polymers, are subject to occasional and unpredictable “wicking in.” This occurs when the edge of the coating moves internal to the perimeter of the substrate. A typical “wicking in” can have a width of several inches or more. In addition, during a continuous coating operation the “wicking in” may continue for several seconds or longer. Often the “wicking in” will continue until corrective adjustments are made to the extrusion coating apparatus. A typical “wicking in” can leave a long section of the substrate inadequately coated. The lack of adequate coverage along the substrate perimeter is a major coating operation defect and must be quickly detected and corrected.
It is known to employ cameras to monitor substrate coating operations. However, the camera systems are expensive and operationally limited. In addition, the camera systems often require a section of the substrate to be shielded or covered for accurate monitoring.
It is also known to use photo sensors to inspect a coated substrate for a given product specification or tolerance. The photo sensors can be used to detect the location of coating edges. Exemplary of such prior art is U.S. Pat. No. 4,357,899 by Jones et al., entitled “Coating Apparatus”. Jones teaches controlling a liquid coating process through reservoir design tanks and a detection system. However, the Jones' detection system is of limited utility. The Jones' system cannot be used on a conventional extrusion coating apparatus for both substrate sheets and a continuous substrate web. In addition, the Jones' system cannot detect gaps between individual sheets or detect a break in a continuous substrate web. Therefore, there exists a need for an improved method and apparatus for detecting coating on a substrate.